Certain phototypesetters employ lenses which are moveable with respect to each other along the optical axis in order to focus and change the size of the images projected upon the photosensitive material. It is desirable to step or drive the moveable carriage in tiny incriments of 1000th of an inch (1 mil) to provide for requisite high degree of accurate lens positioning. After the carriage driving impulse force is removed from the lens carriage, it is necessary to promptly stop the carriage by abruptly overcoming its inertia and maintain the new position. Such action is accomplished by providing a carriage mounted "always on" brake which, regardless of the direction of motion of the carriage, will at all times assert a frictional force upon the moving carriage in opposition to its direction of motion. It was ascertained that a back lash, of 2/1000th of an inch or more was present in commercially available brake designs at the time the invention was made. This back lash of as little as 2/1000th of an inch would reduce the desire sharpness of the projected image or render the image somewhat out of focus.
Bilaterally operating brakes to oppose the inertia of moving members are known in the art. For example, U.S. Pat. No. 2,988,177 issuing June 13, 1961, discloses a spring loaded brake member mounted upon a moveable device which presses against a stationary member to produce forces in opposition to the motion of the moveable member. In FIG. 1 of this patent, central portions of a rigid brake shoe are spring loaded by springs 31 and 32. However, the design inherently requires clearance between the extreme outer portions 33 of the brake shoe and the moving carriage device so that considerable back lash or lost motion would be present, since the springs would be somewhat bent or tipped upon actuation of the brake in a direction transverse their longitudinal axis.
It is thus highly desirable to provide a bi-directional brake of simple design which has virtually no back lash and which provides a constant braking force in either direction, which does not have a tendency to bind, and which is self-adjusting to compensate for wear in order to maintain the requisite bi-directional braking force over the life of the brake.